I'd say a centralist is someone who wants things centralized.

But yes, some people describe themselves as centrist when they mean "I don't view my positions as extreme but neither do I care to justify them".

And some people describe themselves as centrist when they're politically apathetic.

And some people describe themselves as centrist when they have a grab bag of views and don't feel they fit neatly in the right or left wing.

And some people describe themselves as centrist because they're contrary and can't bring themselves to agree with anybody.

And, very rarely, there are people who describe themselves as centrist because they are in the process of changing their mind.